In general, a window installed at a wall of a building is easily polluted by external dusts and air pollution to spoil the beauty and to be worse-lighted. Therefore, it is preferable to frequently clean the window installed at a wall of a building.
However, cleaning an outer side of the window is harder in comparison with cleaning an inner side of the window. Especially, as the buildings become Manhattanized, cleaning the outer side of the window becomes more dangerous.